Radiotherapy is commonly used for treating diseases such as cancer. Various types of radiation sources may be used. The predominant type of radiation used today is photon radiation. Although more expensive than photon radiotherapy, ion based therapy methods, such as proton and carbon therapy, are becoming more common because of their advantages. While photons will pass through a patient's body and emit energy along the whole path, ion beams are subject to very little lateral scattering and can be made to stop at a desired depth in the patient's body. Also, an ion beam will emit an increasing amount of energy as it travels within the body, until it stops. It is possible to plan the treatment in such a way that the ions are made to stop immediately after they pass the desired target area, typically a tumour. Hence, ion radiotherapy may be planned to emit most of its energy in the desired target area and less to surrounding tissue, as compared to photons.
Two main techniques are used for ion radiotherapy: passive therapy and pencil beam scanning. In passive therapy a broad field of radiation is applied and physical elements are used to shape the beam to match the target as precisely as possible. The beam energy will control the maximum range of the beam in the patient. The width of the high dose region, i.e. the Spread Out Bragg Peak (SOBP), is controlled by for example a range modulator or ridge filter. The basic principle is that multiple energies are combined with different weights so that the high dose region is flat, either accomplished by the different steps in a range modulator or by the shape of the ridges in a ridge filter. The other physical elements typically include a block of a non-permeable material such as tungsten or brass, forming a tunnel having the desired shape through which the beam is passed in order to shape it laterally. Another element is the range compensator, which is placed in the beam trajectory to affect the maximum depth of the beam in the patient's body. The compensator shortens the local range of the ion beam and is non-uniformly shaped to compensate for the shape of the tumour. The depth of the target is given as water equivalent depth and depends on the geometrical distance as well as the density and material distribution along the beam path. In areas where the beam should travel a shorter water equivalent depth within the patient, the range compensator will be thicker than in other areas. In pencil-beam scanning a large number of small (pencil) beams are used to cover the tumour in all three dimensions. This is achieved by changing the intensity, position and range of each beam individually. Note that a block and range compensator can be used with the pencil beam scanning technique as well. The main purposes are to sharpen the lateral beam edge (penumbra), and to make each energy layer fit the tumor better, respectively.
One of the main challenges therefore is to design the block and the compensator to shape and modulate the ion beam, so that it will travel the exact distance to the remote end of the tumour along a given beam passing through the tumour. This is traditionally achieved by ray tracing of the patient geometry, which means computing the intersection of the tumour area with the beam coming from the radiation source, taking into account the material composition along the way, and using this information to shape the block and compensator.
WO2008/114159 discloses an apparatus and a method primarily for photon based radiotherapy treatment planning. A treatment planner is used to optimize the treatment based on a weighted combination of treatment objectives, such as dose objectives, treatment device objectives such as the energy provided by the treatment device, and treatment device parameter objectives, such as attenuator or MLC settings, or objectives related to the time rate of change.
Solutions developed for photon based radiotherapy cannot be easily applied to ion based radiotherapy. In particular, the devices used to modulate and shape the beam are different. It should be noted that although devices called compensators are used both for photon based radiotherapy and for ion based radiotherapy these compensators are of fundamentally different nature. The compensator used for photons attenuates the photon beam by removing photons from the beams. The compensator used for ions adjusts the range of the ions, that is, the water equivalent distance they will travel inside the patient's body. This latter type of compensator may therefore be referred to as a range compensator.
For complex patient geometries and tumour shapes, the computation may become very difficult. Some cases may be so complex that it may be practically impossible to create an optimal plan manually. For example, where a critical organ is blocking a portion of the target from the radiation source, a technique called patching may be used, in which different portions of the target are radiated by ion beams from different angles to entirely avoid the critical organ. In such cases, it is difficult to achieve a uniform dose distribution at the border areas between the ion beams. Li et al.: A novel patch-field design using an optimized grid filter for passively scattered proton beams, Phys. Med. Biol. 52 (2007) N265-N275 discloses a patching strategy in which a part of a target is subjected to a through field and another part of the target is subjected to a patch field that is supposed to form a uniform, roughly L-shaped field together with the through field. To compensate for the imperfections in the border area between the patch field and the through field Li et al. propose manipulating the compensator to design the distal fall off of the patch field by means of a grid filter using a repetitive pattern of different material thicknesses of the compensator. This creates a blurring effect at the distant fall-off region, making it possible to match the gradient in the lateral fall off of the main beam. While this may improve the dose uniformity in the border area it will only target this particular case. It will for example not handle cases where the uniformity is different in different parts of the patch region. Furthermore it is not a general method to design the beam energy, range modulator, block and compensator for any beam design.